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package java.lang.invoke;

/**
 * A {@code ConstantCallSite} is a {@link CallSite} whose target is permanent, and can never be
 * changed. An {@code invokedynamic} instruction linked to a {@code ConstantCallSite} is permanently
 * bound to the call site's target.
 *
 * @author John Rose, JSR 292 EG
 */
public class ConstantCallSite extends CallSite {

  private final boolean isFrozen;

  /**
   * Creates a call site with a permanent target.
   *
   * @param target the target to be permanently associated with this call site
   * @throws NullPointerException if the proposed target is null
   */
  public ConstantCallSite(MethodHandle target) {
    super(target);
    isFrozen = true;
  }

  /**
   * Creates a call site with a permanent target, possibly bound to the call site itself.
   * <p>
   * During construction of the call site, the {@code createTargetHook} is invoked to
   * produce the actual target, as if by a call of the form
   * {@code (MethodHandle) createTargetHook.invoke(this)}.
   * <p>
   * Note that user code cannot perform such an action directly in a subclass constructor,
   * since the target must be fixed before the {@code ConstantCallSite} constructor returns.
   * <p>
   * The hook is said to bind the call site to a target method handle,
   * and a typical action would be {@code someTarget.bindTo(this)}.
   * However, the hook is free to take any action whatever,
   * including ignoring the call site and returning a constant target.
   * <p>
   * The result returned by the hook must be a method handle of exactly
   * the same type as the call site.
   * <p>
   * While the hook is being called, the new {@code ConstantCallSite}
   * object is in a partially constructed state.
   * In this state,
   * a call to {@code getTarget}, or any other attempt to use the target,
   * will result in an {@code IllegalStateException}.
   * It is legal at all times to obtain the call site's type using the {@code type} method.
   *
   * @param targetType the type of the method handle to be permanently associated with this call
   * site
   * @param createTargetHook a method handle to invoke (on the call site) to produce the call site's
   * target
   * @throws WrongMethodTypeException if the hook cannot be invoked on the required arguments, or if
   * the target returned by the hook is not of the given {@code targetType}
   * @throws NullPointerException if the hook returns a null value
   * @throws ClassCastException if the hook returns something other than a {@code MethodHandle}
   * @throws Throwable anything else thrown by the hook function
   */
  protected ConstantCallSite(MethodType targetType, MethodHandle createTargetHook)
      throws Throwable {
    super(targetType, createTargetHook);
    isFrozen = true;
  }

  /**
   * Returns the target method of the call site, which behaves
   * like a {@code final} field of the {@code ConstantCallSite}.
   * That is, the target is always the original value passed
   * to the constructor call which created this instance.
   *
   * @return the immutable linkage state of this call site, a constant method handle
   * @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed
   */
  @Override
  public final MethodHandle getTarget() {
    if (!isFrozen) {
      throw new IllegalStateException();
    }
    return target;
  }

  /**
   * Always throws an {@link UnsupportedOperationException}.
   * This kind of call site cannot change its target.
   *
   * @param ignore a new target proposed for the call site, which is ignored
   * @throws UnsupportedOperationException because this kind of call site cannot change its target
   */
  @Override
  public final void setTarget(MethodHandle ignore) {
    throw new UnsupportedOperationException();
  }

  /**
   * Returns this call site's permanent target.
   * Since that target will never change, this is a correct implementation
   * of {@link CallSite#dynamicInvoker CallSite.dynamicInvoker}.
   *
   * @return the immutable linkage state of this call site, a constant method handle
   * @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed
   */
  @Override
  public final MethodHandle dynamicInvoker() {
    return getTarget();
  }
}
